(1) Field of the Invention
The present invention relates to a unit and method for controlling light amplification, and more particularly, to an optical amplifier utilizing the Raman effect and light amplification control unit and method using such an optical amplifier.
(2) Description of the Related Art
Optical amplifiers used in wavelength division multiplexing optical transmission systems include optical amplifiers doped with a rare-earth element (such as erbium-doped optical amplifiers) and Raman amplifiers utilizing the stimulated Raman scattering effect. In the erbium-doped optical amplifier, for example, an erbium-doped fiber amplifier comprising an erbium-doped optical fiber and a pump light source is connected with an external control light generator, and the inverted component within the erbium-doped optical fiber is controlled by the external control light (cf. Japanese Unexamined Patent Publication No. H11-145533 (FIG. 1)).
Meanwhile, if an optical transmission system including optical amplifiers connected in multiple stages is associated with a gain-wavelength characteristic, a low-gain channel is deteriorated in optical signal-to-noise ratio (optical SNR) while in a high-gain channel the waveform is distorted due to nonlinear optical effect etc. It is therefore necessary that the gain-wavelength characteristic should be flattened.
The Raman amplifier uses a plurality of pump light sources for generating beams of different wavelengths and the individual amounts of pump beams are adjusted so that the in-band gain-wavelength characteristic may be constant. For example, there has been known a Raman amplifier which uses pump light obtained by multiplexing beams of 12 different wavelengths, in order to flatten the gain (cf. Y. Emori, et al., “100 nm bandwidth flat gain Raman amplifiers pumped and gain-equalized by 12-wavelength-channel WDM high power laser diodes”, OFC'99, PD19 (1999)).
FIG. 17 shows an exemplary configuration of a conventional Raman amplifier. Light λS incident on an optical fiber 201 is emitted via a WDM (Wavelength Division Multiplexing) coupler 202 and an optical coupler 203. The optical coupler 203 guides part of the incident light λS to a light receiving element 204. The light receiving element 204 receives the light and supplies a control section 205 with an electrical signal corresponding to the intensity of the light. The control section 205 measures the intensity of the incident light λS on the basis of the electrical signal supplied thereto, and computes the intensity of pump light to be input to the optical fiber 201. Then, the control section 205 outputs a control signal indicative of the computation result to a pump light source 206.
The pump light source 206 generates a plurality of pump beams λP1 to λPN with intensity corresponding to the control signal from the control section 205. The pump beams λP1 to λPN generated by the pump light source 206 are introduced into the optical fiber 201 by the WDM coupler 202, whereupon Raman scattering takes place within the optical fiber 201, thus amplifying the incident light λS.
FIG. 18 is a block diagram illustrating a conventional output control method for pump light sources. A signal beam transmitted through a transmission path 210 reaches an optical coupler 212 via a WDM coupler 211. In the optical coupler 212, part of the signal beam is extracted and transmitted to a channel monitor 213. The channel monitor 213 detects the wavelength and output of each channel and supplies, in the form of an electrical signal, information about the wavelength and output of each channel to a control section 214.
The control section 214 determines the output power for each of pump light sources 221, 222, 223. Control information indicative of the determined output power is then supplied from the control section 214 to a corresponding one of the pump light sources 221, 222, 223. Each of the pump light sources 221, 222, 223 outputs a pump beam with power corresponding to the control information supplied from the control section 214.
The pump beams are multiplexed by a multiplexer 231 and transmitted to the WDM coupler 211. The WDM coupler 211 causes the multiplexed pump light from the multiplexer 231 to enter the transmission path 210 in a direction opposite to that of the signal beam.
FIG. 19 illustrates the wavelength-optical output relationship according to the conventional technique, wherein the horizontal axis indicates wavelength and the vertical axis indicates optical output (gain).
As seen from FIG. 19, where the outputs of the individual pump light sources 221, 222, 223 change, the gains of single-pump gain spectra 241, 242, 243 are synthesized, forming a multiple-pump Raman gain spectrum 244.
However, the Raman amplifier needs to be controlled with the gain-wavelength characteristic of light amplification maintained, in order to compensate for level variations of transmitters and loss variations of the fiber transmission path. Also, in the case of a high-gain Raman amplifier operating with high-output pump light sources, if the number of wavelengths changes due to bandwidth upgrade or as a result of increase or decrease in the number of wavelengths by an OADM (optical multiplexer/demultiplexer) node which adds/drops transmit signals, the gain may undergo variation, possibly influencing the signal beam.
In the conventional optical output control wherein the amounts of individual pump beams are adjusted, the pump light sources used for flattening the gain need to be individually controlled taking account of the pump ratio of the light sources, which makes it difficult to control the optical output with ease at high speed. Where the outputs of the pump light sources 221, 222, 223 shown in FIG. 18 change, for example, the single-pump gain spectra 241, 242, 243 overlap each other (cf. FIG. 19). It is therefore necessary that the output information of each channel be monitored to clarify the interrelationship of the individual bands (channels) to be applied, or the power of each pump light source needs to be controlled stepwise. Consequently, it is difficult to have the control section 214 compute suitable power for the individual pump light sources. If the computation is simplified so as to control the pump ratio uniformly, then gain tilt arises.